Heretofore, there have been known various types of position detection devices which employ a coil (inductance element) as a detection element. Many of such position detection devices include a dedicated AC signal source for energization or excitation of the coil, so that the coil is AC-excited by an AC signal generated from the AC signal source being applied to the coil. Examples of the conventionally-known inductive type position detection devices include ones disclosed in Patent Literature 1 and Patent Literature 2. The inductive type position detection devices disclosed in Patent Literature 1 and Patent Literature 2 include a primary coil and a secondary coil and are constructed in such a manner that the primary coil is excited by an AC signal and a secondary output signal responsive to the primary coil excitation is induced in the secondary coil. Further, in the inductive type position detection devices disclosed in Patent Literature 1 and Patent Literature 2, inductance of the secondary coil is varied in response to movement or displacement, relative to the secondary coil, of a magnetically responsive member (e.g., iron or copper) that is displaced in response to a position to be detected (i.e., position of a target of detection), so that an output signal corresponding to the position of the target of detection. In this case, an oscillation circuit that generates an AC signal for exciting the primary coil is provided separately from the coils. Also known are proximity sensors which can eliminate the need for, or dispense with, a dedicated exciting AC signal source by use of the principle of an LC oscillation circuit, i.e. by incorporating a coil, functioning as a detection element, in a self-oscillation circuit as an inductance element (see, for example, Patent Literature 3). Such self-oscillation type proximity sensors are advantageous in that they can be significantly reduced in size because the need for providing a dedicated exciting AC signal source can be eliminated. However, because the conventionally-known self-oscillation type proximity sensors are constructed to detect variation in oscillating frequency responsive to proximity of a target of detection, they require a frequency discrimination circuit. Further, although the conventionally-known self-oscillation type proximity sensors are constructed to be suited for detection of variation in oscillating frequency, they are not constructed to be capable of detecting a position of the target of detection on the basis of an amplitude level of an oscillation output signal.
If a challenge to reduction in size of the device construction is considered from another perspective, there can be employed an approach of using, as the detection element, a small flat coil formed in a spiral shape on a printed circuit board, one example of which is disclosed in Patent Literature 2. In position detection devices using such a flat coil, the number of winding turns of the coil is so small that it is difficult to obtain a sufficient magnetic flux for detection. To make up for or supplement such an insufficient magnetic flux, a device disclosed in Patent Literature 4 employs a technique of providing the flat coil in multiple layers.